| Indeed, all fair points. Though I disagree that Hróbjartsson et al undermines the notion of many placebo effects.. or maybe I don't, depending on our definition of placebo effect. If we define "placebo effect" as any clinically significant change observed in the placebo wing of a clinical trial, there are many placebo effects. As I mentioned before, regression to the mean, natural history of the disease, experimenter effect, and so on. Alternatively, we could define it as any clinically significant change observed in the placebo wing of a clinical trial which is not also observed in a no treatment wing. This would eliminate things like regression to the mean and arguably leave behind only a "true" placebo effect. Though even here, we have to account for bias (and perhaps even classical conditioning?) I wonder how much placebo effect is left if these are controlled for? Within the context of analysing trial data, it is the former definition we are interested in. But within the context of discussing "the placebo effect" as a standalone phenomenon, I'd argue the latter definition is more useful. Unfortunately, as most trials don't have no treatment arms, any clinically significant changes observed in the placebo arm are chalked up to "the placebo effect", especially by the media (though sometimes by clinicians), even when there is good reason to think many of those same changes would have been observed under no treatment. Which IMHO leads to a distorted view of the clinical relevance of the placebo effect outside the context of a clinical trial. Especially as the placebo effect appears to have a reputation as a bizarre mind-over-matter affair. |
In terms of what's left over after accounting for no treatment, Vase and H&G got into a big academic fight about this, and it appears that the effect size for placebo effects in pain is approximately (d=0.5), which is a relatively large effect (especially within psychology). This doesn't entirely account for experimenter effects, though with the use of a balanced-placebo design, those can be accounted for.
In terms of clinical trials, I would tend to agree with your second definition. Its not perfect, but its as good as it tends to get.
I think that one of the issues with placebo research is this notion of mind-over-matter, in that such a viewpoint is the reason that it is perceived as special, and also a reason why people disbelieve in it.
Based on old research (Levine, 1979) many (but not all) placebo effects in pain appear to be mediated by endogenous opioids,which I would take to mean that they are pretty naturally mediated by the brain, and so its a physical phenomenon. Many people do go a bit crazy with the woo around it though, I do agree.
Funny that this came up today, when I'm currently finalising a hopefully final draft of my thesis on the placebo (PROTIP: never, ever leave your university before submitting a PhD, it tends to go badly).